Looking for some advice.
Very experienced lightweight backpacker, mostly in the rockies.
One piece of gear that I have resisted changing out was a cold-weather bag. Have a WM Highlite for the high summer/lower elevations...

But this year I will be around treeline in CO in early september. Certainly a chance of some snow, and a night down around 20 would be a real possibility.
Basically, I am trying to decide on a new bag, going as light as possible...but I want to be warm.

So...20 degree bag, 15 degree or 10 degree? Budget not the concern - weight and comfort are.

Will be paired with a thermarest xtherm which has a 5.7 r-value.

If it matters, I am in a Stephenson 2R. Because I like condensation.

Thoughts?